1. Field of the Invention
The present invention relates to an electrically conductive multilayered film, and a transparent electrode substrate and a liquid crystal display device using the same, and more particularly to a multilayered conductive film having a high storage stability, and a transparent electrode substrate and a liquid crystal display device, using such a multilayered conductive film.
2. Description of the Related Art
An electrode substrate having a transparent electrode film or a light-reflecting electrode film formed on a substrate such as glass or plastic film is widely used as a displaying electrode for various display devices such as liquid crystal display devices, and as an input-output electrode for directly inputting power thereinto from the display image screen.
For example, a transparent electrode substrate has a glass substrate, and a color filter layer which is formed at the pixel sites, and colors each light transmitted through each pixel in red, green and blue. At the sites (inter-pixel sites) between the pixels, a light-shielding film is formed which prevents the light from transmitting through these inter-pixel sites. On the entire surface of the color filter layer, a protective film is formed, and a transparent electrode is formed on the protective film. An orientation film is formed on the transparent electrode. The transparent electrode is made of an electrically conductive transparent film which is formed by sputtering technique and is etched into the predetermined electrode pattern.
As the transparent conductive film, a thin film of ITO, indium oxide added with tin oxide, is widely utilized because of its high electrical conductivity. It has a resistivity of about 2.4.times.10.sup.-4 .OMEGA..multidot.cm, and exhibits a sheet resistance of about 10.OMEGA./square when it has a thickness of about 240 nm, a thickness which is usually applied as a transparent electrode.
In addition to the ITO film, thin films of tin oxide, of tin oxide added with antimony oxide (NESA film), and of zinc oxide added with antimony oxide are known. However, these films are inferior to the ITO film in conductivity, and have an insufficient resistance to chemicals such as acids and alkalis, and an insufficient water or moisture resistance. Therefore, these films are not widely used.
Recently, the above-mentioned display devices and input-output devices have been required of displaying fine images by increasing the density of pixels. Thus, it is required that the transparent electrode pattern have a high density. For example, it is required that the terminals of the transparent electrode be formed at a pitch of about 100 .mu.m. Further, in the system (COG) in which an integrated circuit (IC) is directly connected to the substrate, the wiring sometimes has fine portions having a width of 20 to 50 .mu.m, and thus it is required that the electrode material have so high etching processability and so high conductivity (low resistivity) that have not been required in the past. The ITO material can not meet these requirements.
On the other hand, a larger display screen is required. It is necessary to use, as the transparent electrode, a transparent conductive film having a high conductivity such that it exhibits a sheet resistance of 5.OMEGA./square, in order that a transparent electrode of fine pattern as mentioned above can be formed and a sufficient driving voltage is applied to the liquid crystal. In addition, a sheet resistance as low as 3.OMEGA./square or less is required when a multi-gradation display of 16 or more gradations is effected in a liquid crystal display device of a simple matrix driving system, utilizing, for example, an STN mode liquid crystal materials. The ITO material can not meet these requirements, either.
Meanwhile, silver has the highest conductivity among metals, and ensures a sufficient transparency and a sufficient conductivity even when formed into a thin film. For example, at a thickness of 5 to 30 nm, silver exhibits a transparency sufficient to transmit visible light, and a sheet resistance of about 2 to 5.OMEGA./square. Accordingly, silver is a promising material as a conductive material which meets the low resistivity requirements mentioned above.
However, silver is damaged in about one week when allowed to stand at room temperature in air. More specifically, silver reacts with sulfur compounds and water present in air to form the sulfide and the oxide on its surface, and is thus deteriorated. For these reasons, silver is not commonly used as a light-reflecting electrode in a reflective type liquid crystal display device, or a light-reflecting substrate, either, although silver may have a reflectivity higher than aluminum and may establish a high contrast display on the screen.
A transparent conductive film of a three-layered structure in which an ITO thin layer or an indium oxide (IO) thin layer is formed on each surface of a silver thin layer has been proposed in Published Unexamined Japanese Patent Application (Kokai) Nos. 63-173395, 1-12663 and 2-37326, and The 7th ICVM held in Japan, 1982. This three-layered transparent conductive film has a low sheet resistance of about 5.OMEGA./square, and has been hoped for the application to the transparent electrode.
However, even in this conductive film of three-layered structure, the silver thin layer reacts with the water entering at the interfaces between the layers to form the oxide on its surface, creating stain-like defects which lead to display insufficiency on the screen when the film is applied as the transparent electrode in a liquid crystal display device, for example.